Conventionally, there is proposed a technique for arranging two superchargers (primary turbocharger and secondary turbocharger) in parallel with each other on an intake system and an exhaust system and for suitably switching the number of the superchargers in operation. Concretely, there is proposed a control which is performed in case of switching between a mode in which only the primary turbocharger is operated and a mode in which both the primary turbocharger and the secondary turbocharger are operated. Hereinafter, the mode in which only the primary turbocharger is operated will be called as “single turbo mode”, and the mode in which both the primary turbocharger and the secondary turbocharger are operated will be called as “twin turbo mode”.
For example, in Patent Reference-1, there is proposed a technique for determining whether or not an acceleration state is an rapid acceleration based on a number of engine revolution and a throttle opening degree and for making a switching timing earlier at the time of switching from the single turbo mode to the twin turbo, in case of determining that the acceleration state is the rapid acceleration. Additionally, in Patent Reference-2, there is proposed a technique for determining the switching between the single turbo mode and the twin turbo mode, based on a difference between a target supercharging pressure in the single turbo mode set by a driving state and a target supercharging pressure in the twin turbo mode.
Patent Reference-1: Japanese Patent Application Laid-open under No. 5-98976
Patent Reference-2: Japanese Patent Application Laid-open under No. 2008-128129